Cost, plain and simple. Cost of the tooling but especially the cost of labour as I don't think you could automate the process. And the fact that making/doping the plates etc requires the use of some seriously hazardous/environmentally unfriendly chemicals. Couple that to an altogether limited market for 6AS7(G)/6080 and the fact that there are still NOS versions available with all the associated nostalgia and premium those will command even if the newly produced ones are objectively better.
Apparently it's worth it for the more expensive/rare tubes such as the 300B if you can produce them in a low wage country. The present owners of the Tung Sol brand currently even produce 6SN7 I believe....but it's a volume thing with that tube. You need low production cost and either high margin and/or high volumes and the tubes we're talking about don't seem to offer that.